Freestyle Lines on Wrong Frames
I can think of two things that you might have done:


You have Motion Blur switched on as suggested by Mechanic
You have used a transparent material to hide it.


I can't say anything else if you don't provide the blender file through https://blend-exchange.giantcowfilms.com/. With the actual file I would be able to troubleshoot your problem effectively cause I can't see your settings and stuff through the two images.